In the semiconductor industry, there is a continuing effort to increase device density by scaling device size. Conventionally, to form an integrated circuit, a photoresist layer is formed on a wafer and is exposed to radiation through a photomask (“mask”). A mask typically comprises a substantially transparent base material such as quartz with an opaque layer having the desired pattern formed thereon. For example, chrome has long been used to make the opaque layer. When device features are reduced to a dimension below 1 micro level, diffraction effects become significant. The blending of two diffraction patterns associated with features which are close to each other has an adverse effect on resolution. As a result, portions of the photoresist layer underlying the opaque layer near the edges of features will be exposed.
To minimize effects of diffraction, phase shifting masks have been used. Typically, a phase shifting mask has a pattern in the opaque layer for transmitting exploring radiation which corresponds to the pattern to be formed on the underlying photoresist. Phase-shifters, which transmit the exposing radiation and shift the phase of the radiation approximately 180 degrees, are added to reduce diffraction effects. Alternate aperture phase shifting masks are formed by adding phase-shifters over every other opening. In rim phase shifting masks, phase-shifters are added along or near the outer edges of features. The radiation transmitted through the phase-shifter destructively interferes with radiation from the opening, thereby reducing the intensity of radiation incident on the photoresist surface underlying the opaque layer near a feature edge to improve image resolution.
Such phase shifting masks, however, have limitations on their ability to pattern some features and are difficult to fabricate. When two features such as two thin lines are placed in close proximity to each other in rim phase shifting masks, it is difficult to clearly form such features on the substrate because merger of associated phase-shifters causes over exposure to the region of photoresist between them. Further, phase-shifters may be fabricated by a separate step from the formation of the pattern on the opaque layer. To improve resolution by destructive interference, the locations of the phase-shifters must be precisely correlated with the pattern on the opaque layer. For very small features, the alignment tolerance between the opaque layer with pattern and phase-shifters may exceed the capability of the process.
To resolve these problems, an attenuated phase-shifted mask (“AttPSM”) has been proposed. The AttPSM replaces the opaque layer (which is typically a layer of chrome about 0.1μ thick) with a “leaky” layer which transmits a reduced percentage of the incident radiation. For example, a very thin layer of chrome (approximately 300 angstroms) with approximately 10% transmittance could be used as the leaky layer. In addition, the leaky chrome layer shifts the phase of the transmitted radiation by a certain number of degrees, for example approximately 30 degrees, depending on the thickness and refractive index of the layer. To achieve the required 180 degrees phase shift between radiation transmitted through regions covered by the leaky chrome layer and regions of features, the features are also phase shifted a complementary angle by etching the mask or by placing a phase-shifting material in the regions of features.
Nonetheless, it is extremely difficult to deposit a thin layer of chrome with uniform thickness across the surface of the mask. Furthermore, physical characteristics such as refractive index fluctuate across the surface of the leaky chrome layer on the mask. The leaky chrome layer itself can not shift the phase of incident radiation 180 degrees. Additional processes needed to achieve this goal increase manufacturing cost and complexity.
To overcome these difficulties, an embedded coating material which integrates the property of obtaining the required phase shift—180 degrees—into the substrate coating layer which transmits a reduced percentage of the incident radiation, has been used. An embedded coating material such as molybdenum silicide (MoSiOxNy) is used to achieve AttPSM. However, molybdenum silicide only provides a low transmittance of about 8 percent. An embedded material which can attain high transmittance and is chemically stable under repeated use in the DUV exposure environment is needed.